Conventional electronic blood pressure meters are mostly based on an indirect method in which a cuff is wrapped around an upper arm of a patient and pressurized to obstruct the blood flow in the upper arm, and the cuff pressures corresponding to the time points of appearance and disappearance of Korotkoff sound during the process of depressurizing the cuff are determined as the systolic (maximum) blood pressure and the diastolic (minimum) blood pressure, respectively.
An electronic blood pressure meter based on Riva-Rocci-Korotkoff method however has the disadvantage that since the Korotkoff sound is to be picked up by a microphone an accurate measurement of blood pressure is sometimes impossible particularly when the surrounding is noisy or when the cuff is rubbed by an object and the resulting sound is picked up by the microphone.
According to another method of measuring blood pressure or so-called oscillation method, the pulse wave produced in a living body in synchronization with the pumping motion of a heart is measured and blood pressure values are computed using the amplitude of the pulse wave as a parameter according to a certain algorithm.
Since this method does not require a microphone to pick up the pulse wave from an artery, the above-mentioned problems of the Riva-Rocci-Korotkoff method would not occur but the oscillation method still requires to apply a cuff to one's upper arm and it is quite cumbersome that the patient must roll up his sleeve for blood pressure measurement.
In view of such inconvenience of the prior art, the present inventors have realized that the above mentioned problems will be eliminated if an accurate blood pressure measurement can be performed on a part of a body which is normally exposed, such as a finger.
A certain device is known according to which water is used for pressing a finger for the purpose of measuring blood pressure from an artery in the finger, but a roller pump is necessary for the pressure control of the water which fills a cuff for the application or pressure to the finger and must be provided separate from the main unit, making it impossible to achieve a desired compactness of the structure.
Electronic blood pressure meters using air cuffs for pressurizing one's upper arm are well known in the art but an electronic blood pressure meter using such a cuff can not be directly applied to measuring blood pressure by one's finger since the volume of the cuff and the venting speed are excessive and the pressurization unit and, therefore, the signal processing unit of a conventional electronic blood pressure meter are unsuitable for this application.
For instance, a typical air cuff for a conventional blood pressure meter consists of a rectangular flexible air bag having an outer and an inner skin having the same dimensions and, therefore, when it is wrapped around one's arm and inflated by air pressure, the inner skin tends to develop creases or folds thereby causing uneven application of pressure to the upper arm. This tendency becomes more pronounced as the cuff is wrapped around an object having a smaller diameter such as a finger. Furthermore, the orientation of a sensor device attached to the inner skin tends to be unpredictable if such folds are produced in the vicinity of the sensor, thus reducing the reliability of the sensor.
In measuring blood pressure, it is necessary to detect the pulse wave of an artery but the artery in one's finger is so fine that a conventional pulse wave detector is not adequate for accurate detection. In a conventional pulse wave detector, as shown in FIG. 6, light emitted from a light emitting element L (for instance an LED) through an artery D in a finger F is received by a light sensitive element PT (for instance a photo transistor) and the pulse wave of the artery is detected as the changes of the intensity of the light received by the light sensitive element PT. According to this detector, since the light must pass through a distance corresponding to the width of the finger, it is difficult to achieve a desired sensitivity and the signal to noise ratio (SN ratio) of the signal detected by the light sensitive element PT tends to be low.
Another shortcoming of the above-mentioned conventional methods of measuring blood pressure is that since the measurement process takes place as the air pressure is gradually reduced and a substantial pressure must be built up in the cuff prior to starting the measurement the patient is subjected to a discomfort for a substantial time period.